This invention relates to rotating anode x-ray tubes wherein the target and anode assembly rotate in ball bearings at high speed.
There are two basic types of rotating anode x-ray tubes insofar as arrangement of the structural components is concerned. In the first type, which is the type used for illustration herein, a metal sleeve is mounted in a vacuum tight fashion in the x-ray tube envelope and one end of the sleeve extends from the envelope for allowing an external electrical connection to be made to it. The outer races of two axially spaced apart ball bearings are mounted in opposite ends of the sleeve. A shaft is supported at its opposite ends in the inner races of the axially spaced apart bearings. An outer sleeve that is concentric with earlier mentioned sleeve and constitutes the rotor of an induction motor is provided with an axially extending stem on which the x-ray tube target is fastened. The outer rotating sleeve is usually coated with a material that has high heat emissivity to dissipate as much as possible of the heat that is developed in the target as a result of the electron beam of the tube striking the target for the purpose of generating x radiation. A substantial amount of heat is conducted through the bearings so that under expected operating conditions bearing temperatures may be on the order of 500.degree. C. The inner and outer races of the bearings and the balls are coated with silver which constitutes the lubricant for use in the high vacuum and high temperature environment that exists in an x-ray tube for a normal operation. For some x-ray protocols the anode and target only have to be rotated at around 3600 rpm to avoid melting of the target at the x-ray beam focal spot and for other protocols where higher x-ray tube currents and voltages are used, the target is customarily rotated at about 10,000 rpm. Typically, the rotor is driven as a two-pole induction motor so that 50 or 60 Hz is applied to the field coils for the lower speed and 180 Hz is applied for the higher speed. In foreign countries, the frequencies might be 50 Hz and 150 Hz, respectively.
A second type of rotating anode x-ray tube is the structural converse of the first one. In the second type, instead of having a rotating shaft, the shaft is fixed. There are axially spaced apart bearings on the shaft. A sleeve which has the stem extending from it and that supports the target, fits tightly on the outer races of the bearings so it is the outer races that turn rather than the inner races as in the first case. In the second case, as in the first, the bearings act as thermal conductors and as conductors of electricity as well.
In both types of tubes, the front ball bearing, that is, the bearing nearest to the target is loaded radially and in cantilever fashion by the heavy target suspended at the end of the stem. The center of gravity is invariably between the front bearing and target somewhere along the stem so the radial load on the front bearing is greater than that on the rear bearing which is axially displaced from the front bearing. The radial reactive forces on the front and rear bearings are opposite of each other in both types of rotating anode x-ray tubes.
All modern prior art heavy duty rotating anode x-ray tubes of which applicants are aware use the same kind of ball bearings which differ from bearings used in the invention described herein. Typically, the outer race of the prior art ball bearings is either flat or has an angular groove whose cross-section constitutes a segment of a circle in which the balls run. The inner race has an angular groove that is basically v-shaped in cross-section. More specifically, the inner race groove has a cross-section that is more analogous to a modified gothic arch. The arch configuration is similar to what would be obtained if an inner-race ring had a groove machined in it that coincided in cross-section with an arc or segment of a circle. Then, by doing the equivalent of sawing the ring in half and removing some material between the two ring sections that remained and then pushing the sections into interfacing relationship a more nearly v-shaped or gothic arch shaped groove will result. In reality, of course, the groove is formed in a single machining operation. When this prior art bearing is assembled, the balls make 3-point contact with the races. Two points of contact are made where the ball is tangential to the slanted grooves in the halves of the inner race and one contact point is made between the balls and the outer race. In anticipation of the high temperatures at which the x-ray tube will operate, a certain amount of clearance has been provided between the balls and races to account for the fact that races and balls will expand substantially as a result of becoming hot during use of the tube. If no clearance were allowed, the bearings would seize rather quickly when they got hot. Moreover, if clearance is too small any silver particle that flakes off of the rolling surfaces of the bearings or balls can wedge between the balls and races and freeze the bearing. On the other hand, if the clearance is too great the bearing may run noisily and the target may wobble so as to oscillate the focal spot of the tube which militates agains obtaining sharp radiographs. There are two other disadvantages to trying to avoid bearing seizure by using a substantial amount of clearance between balls and races. One disadvantage is that the balls are more free to bounce in which case, since they are conducting electricity, there will be sparking between the balls and races which will result in their roughening and premature failure. Another disadvantage of excessive clearance between the balls and races is that the cantilever loaded shaft or sleeve that rotates will exhibit excessive deflection which ultimately results in only one ball at a time in the front bearing being radially loaded and a diametrically opposite ball in the rear bearing being radially loaded at the same time. Hence, the one ball in each bearing that is loaded at any moment is subjected to excessive stresses which will cause it to fatigue and cause premature bearing failure. An ideal bearing is one in which all of the balls accept or share the radial load equally at all operating temperatures and all angles of rotation of the x-ray tube rotor. This has never been achieved in an x-ray tube before the invention described herein was made insofar as applicants are aware.
The stratagem used in the x-ray tube described herein to obtain more uniform load sharing by the ball bearings is to actually, in a sense, increase the loading on them by applying a substantial axially directed force on the bearing races and, thus, to the balls.
Pseudo-axial preloading of ball bearings in a rotating anode x-ray tube has been done heretofore as in U.S. Pat. No. 4,272,696 which is assigned to the assignee of the present application. The objective of the inventors in the patent was to keep the bearing balls in contact with the surfaces of the grooves in races to avoid sparking that would occur between the balls and races and which was thought to roughen the bearings and cause premature failure. The approach in the prior patent was to apply an axial load to corresponding races on the front and rear bearings to thereby press the balls axially and force them to make good contact with the other races. The axial force was obtained by disposing a coil type prestressed compression spring made of molybdenum between corresponding races. Conventional three contact point bearings were, of course, used. These bearings had a total clearance of about 0.0015" between the balls and races. The life of the bearings was, however, not extended as much as what was expected. Hence, the axial loading spring was redesigned to raise the axial force from 1.5 lbs. to 3 lbs. Since there was 3-point ball contact and a greater axial load, bearing friction simply increased and the bearings got hotter and expanded and ultimately froze. The conclusion was that in the particular x-ray tube design, given bearings and shaft of a certain size, the axial force had to be under 3 lbs. Yet at this lower level of axial loading, the disadvantage of having one ball in each bearing carrying most or all of the radial load was not overcome.
Rotating anode x-ray tubes are regularly used in computed tomography apparatus. The targets in x-ray tubes used in this apparatus are usually composed of tungsten-rhenium alloy on a molybdenum substrate. Since the targets must have substantial thermal capacity they may have a diameter of about 5" (12.7 cm) and such thickness as to create a radial force of over 5 lbs. (2.27 kg) on the front ball bearing of the tube. The target is mounted on the free end of a stem whose other end is fixed to the rotor so there is a substantial cantilever force as well as radial force applied to the front bearing in particular. In computed tomography apparatus, the x-ray tube is mounted on a scanner carriage which rotates to cause the tube to orbit around a patient through an angle of 360.degree. or more for making an x-ray scan of a layer in a body. The scanner carriage rotates on a tilting gantry so the scanning plane can be set at an angle of up to 20.degree. from vertical to permit scanning a body layer at an angle. During the era when the x-ray tube described in the cited patent was devised, the time for making a full circle tomographic scan was typically about 8 seconds or a little less. The scanning time in the currently most advanced computed tomography apparatus has been reduced to a little more than 2 seconds. The rotational axis of the x-ray tube anode is parallel to the orbit axis during a scan. When the gantry is tilted, however, gravitational (g) forces acting on the anode of the tube are quite high and the anode and target tend to undergo precession which imposes even greater radial forces on balls of the bearings to thereby increase the stress on the one ball in each bearing in prior designs that took all of the load. It may also be noted that any unbalance in the target, especially, results in adding to the radial load on the balls of the bearings.
Axial preloading of ball bearings has been employed in rotating machinery outside of the x-ray tube field art. In fact, equations and computer programs have been developed for determining the amount of axial preload force required for getting all of the balls in bearings on a common shaft to share the radial load substantially equally. The generally accepted equation for the minimum preload force that is required to obtain load sharing by the balls, states that the preload force is equal to the radial load multiplied by the tangent of the angle which the balls make with the sloping race surfaces. For example, it has been the practice to preload bearings in aircraft turbine engines to assure that the turbine shaft and its blades will remain centered regardless at the rates at which the races and balls of the bearings heat and expand and regardless of the temperature differential between parts of the bearings. In this kind of application, large bearings are used and they are loaded radially until they are rather close to their permissible stress with some margin of safety being allowed. In such applications, and in x-ray tubes as well, excessive radial stress and, particularly, where most of the radial load is transferred from one ball to the other during rotation, loading and unloading a ball near its maximum permissible stress can result in fatigue of the metal balls and fracture or permanent deformation. In any event, if enough axial preloading is used to make sure that the rotating part remains centered and thermally compensated despite the fact that as axial preloading is increased, bearing friction also increases. Even though this is true, tests have shown that the axially preloaded bearings of the present invention wherein the balls make two-point contact with the races there is substantially less friction than in conventional 3-point contacting gothic arch type of bearings races.